Recently, the downsizing and performance improvement of the electric devices are under rapid development. Also, the improvement on the transmission speed and lowering driving voltage of the used circuit element are remarkable, as seen in the high-speed transmission systems such as the antenna circuit of the mobile phone, RF module, USB 2.0 and USB3.0, S-ATA2, HDMI or the like. With the downsizing of the electric devices and lowering driving voltage of the circuit element, the breakdown voltage of the electric parts which are used in the electric devices is decreased. In this respect, the protection of the electric components from overvoltage becomes an important technical subject, for example, protecting the electric components against the electrostatic pulses derived from the contact between the human body and the terminal of an electric device.
In the past, a laminated varistor is disposed between entrance line of the static electricity and the ground to protect the electric components against such electrostatic pulses. However, the laminated varistor usually has a large electrostatic capacity, which is the main reason that makes the signal quality deteriorate when a high-speed transmission system is used. In addition, an electrostatic protection device with a large electrostatic capacity cannot be used in an antenna circuit or a RF module. Thus, an electrostatic protection device with a low electrostatic capacity which is applicable to the high-speed transmission system needs to be developed.
It has been suggested that an electrostatic protection device with a low electrostatic capacity may has separate electrodes arranged oppositely and may use the composite (which is disposed between the electrodes) of the conductive inorganic materials and insulating inorganic materials as the discharge triggered part. Similar to the laminated varistor, such an electrostatic protection device is disposed between the static electricity entrance line and the ground. If a much too high voltage is applied, discharge will happen between the electrodes of the electrostatic protection device and then the static electricity will be led to the ground side.
The electrostatic protection device provided with gap typed electrodes has characteristics such as large insulation resistance, small electrostatic capacity and good responsiveness. On the other hand, a problem exists that short may occur because of the agglomeration of the electrodes and conductive inorganic materials inside the discharge triggered part due to the heat or stress generated during the discharging process.
In order to prevent short due to the discharging, for example, a technical solution has been provided as disclosed in Patent Document 1. Specifically, the electrostatic protection device disclosed in Patent Document 1 is characterized in that metal oxides such as the oxides of aluminium or magnesium are attached to the surfaces of the discharge electrodes in the gap typed discharge elements which have a pair of discharge electrodes at the hollow portion. With such a structure, the short between the oppositely arranged discharge electrodes can be inhibited since the oxides attached to the discharge electrodes have high insulation resistances. Also, the durability against repeated applying of static electricity can be improved.